Distributed automatic gain control system

[0001] The present invention is related to a high capacity distributed communications system, and more particularly to a distributed automatic gain control (AGC) system for use in a point-to-multipoint communication system in which wireless (radio frequency or otherwise) signals are distributed in buildings or other areas where wireless signal propagation is likely to be a problem.

[0002] The operational range of a system that combines and distributes signals within buildings or other areas where wireless signal propagation is likely to be a problem, whether the signals are digital or analog or some combination of both, is limited by the dynamic range handling capability of the system. In a distributed system, such as the one disclosed in U.S. patent application Ser. No. 09/619,431, “Point-To-Point Digital Radio Frequency Transport,” filed on Jul. 19, 2000, incorporated herein by reference as if fully set forth, signals of varying levels are present at input ports, at signal combiners and at output ports. A large signal at a single input port may potentially saturate the output port (e.g., cause an overflow condition) if the large signal exceeds the dynamic range of the system and is not controlled in some way.

[0003] Even if a saturation level is not reached, a large signal may effectively limit the amount of traffic that the system can handle. For example, capacity in a spread spectrum system, such as a system employing a spreading function (for example, code division multiple access (CDMA), IEEE 802.11 complimentary code keying (CCK), or the like) is generally limited by the total amount of power allowed for all users in the bandwidth of interest in order to prevent unacceptable interference. Thus, power levels of spread spectrum mobile unit transmitters are continually adjusted to a level that is sufficient to maintain good signal reception at the base station but also minimizes signal power levels. For signals from mobile units that are supported by the distributed point-to-multipoint digital micro-cellular communication system (distributed communication system), power level adjustment works seamlessly. For example, if the signal from a supported mobile unit is boosted and provided to a base station, the base station will inform the mobile unit that its transmitter power should be correspondingly reduced. However, the frequency spectrum that is received, processed and distributed by the distributed communication system is shared by many services and devices and thus the distributed communication system will likely receive and process signals from communication services that are not supported by the system as well as from other sources of interference in the band. For example, the communications standard for wireless local networks, IEEE 802.11, calls for using the 2.4 GHz Industrial Scientific Medical (ISM) band. The 2.4 GHz ISM band has been called the “junk band” because it is contaminated by microwave oven emissions. Such non-supported and interfering signals may reach significant power levels in the coverage area and cause significant problems.

[0004] For example, a distributed communication system in a building or other enclosure may share a coverage area with a non-supported service. Further, the non-supported service may have mobile units in the coverage area that share the same bandwidth with the distributed communication system. Unfortunately, the mobile units from the non-supported service may transmit at much higher power levels compared to the distributed communication system to provide for good reception by the non-supported base station, which may be located a significant distance from the building or enclosure. This power level may be many orders of magnitude greater than signals from supported sources that are much closer to radio transceivers in the building or enclosure and may effectively “power limit” the capacity of the system or saturate the distributed communications system.

[0005] The above-mentioned problems with controlling the gain of wireless signals in distributed communication systems within buildings and other enclosed areas, as well as other problems, are addressed by the present invention and will be understood by reading and studying the following specification.

[0008] A wireless distribution system according to one aspect of the present invention includes a plurality of remote units distributed in a coverage area to receive wireless signals in the coverage area and provide the signals to a plurality of input ports to receive signals comprising the signals provided by the plurality of remote units a plurality of input power monitors operatively connected to one or more of the plurality of input ports to determine power levels of signals received at the input port a plurality of variable gain controllers to control the gain of signals received at the one or more of the plurality of input ports in response to a control signal, a node to combine a plurality of signals from the plurality of input ports, and a controller to provide control signals to individually control each of the variable gain controllers.

[0009] In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention.

[0010] FIG. 1 is an illustration of a representative system 100 for distribution of wireless signals in a difficult environment for wireless signal propagation, such as in a large building or enclosure. The present invention is not limited to wireless signal propagation in and around buildings or other enclosures but is applicable to enhanced wireless systems for any coverage area. The system 100 includes a number of digital remote units (DRUs) 102_i to 102_n that receive a frequency spectrum of wireless signals such as may be transmitted by low power wireless devices in and around the building. In general, the DRUs process the spectrum of wireless signals so that the spectrum can be transmitted over transmission links 104_i to 104_n, which may include one or more transmission media such as fiber optic, coaxial, twisted pair or simple copper wire, wireless link, or other medium of information transmission, and interface devices for such media or combinations thereof. The signals may be processed before, during and after transmission over the transmission links 104_i to 104_n to improve signal characteristics and propagation over the transmission media. Processing of the signals may include analog to digital conversion, analog and digital filtering, mixing and frequency translation, amplification and other well-known signal processing techniques. The transmission links 104_i to 104_n terminate at a node 110 where the signals are combined. Signals combined at node 110 can originate from the various DRUs 102 as well as from other upstream nodes such as digital extension units (DEUs), which also combine signals from DRUs or other DEUs.

[0011] In general, the frequency spectrum served by distribution system 100 is shared by a number of users and services. Users and services that are supported by distribution system 100 will usually be subject to and will abide by power control limitations of the system so that signal interference will not result from the use of excessive power. However, not all of the users and services sharing the frequency spectrum are supported by distribution system 100 and some of the signals, particularly those that are not subject to the power control limitations imposed by supported services may reach sufficient amplitude such that they may interfere with signals supported by the distribution system. This is particularly likely if strong signals are broadcast in close proximity to a receiving unit such as DRUs 102_i to 102_n. The distributed wireless communication system has a limited dynamic range and accordingly must be protected from signals that would saturate or otherwise exceed the dynamic range handling capability of the system.

[0012] One example of an AGC circuit for use in a distributed communication system according to the present invention is shown in FIG. 1. DEU 100 includes a node 110 for summing signals received on transmission links 104_i to 104_n. AGC circuit 106 is inserted upstream of node 110 in order to protect the transmission system from reaching saturation levels. AGC circuit 106 senses a power level at power sensor 108 in transmission link 105 and feeds back a gain control signal by control 114 to node 112 where gain control is applied to the summed signals on transmission link 105. In this way, gain control is provided upstream from node 110 so that an overflow condition is avoided. While the system of FIG. 1 shows only one DEU, it should be understood that many such DEUs may be included in a large distributed communication system.

[0013] FIG. 2 is an illustration of one additional example of a distributed automatic gain system 200 according to the present invention. System 200 controls the gain of signals distributed in a multiple point to point wireless system to enhance signal coverage, particularly in areas such as buildings and other enclosures where wireless signal coverage is a problem. The system 200 includes a number of digital remote units 202_i to 202_n that receive wireless signals and process them for delivery upstream over transmission links 204_i to 204_n. In general such transmission links may be fiber optic, coaxial, twisted pair, wireless, or other medium of information transmission, or combination thereof. Each DRU 202_i to 202_n includes an input port or receiver that receives, processes and digitizes a wireless bandwidth. In one example each DRU receives the same wireless bandwidth. The DRUs 202_i to 202_n transmit the digitized signals upstream to a digital expansion unit (DEU) or to a digital host unit (DHU) represented by summing junction 226. Summing junction 226 sends the summed digitized signals to another DEU or a digital host unit 228 (DHU) located further upstream.

[0014] DEU 240 has a number of input ports 205_i to 205_n for receiving transmission links 204_i to 204_n from each DRU or DEU located upstream from DEU 240. Each input port 205_i to 205_n has associated with it an input power level calculator 208_i to 208_n to determine power levels of signals received over transmission links 204_i to 204_n. The input power level calculators 208_i to 208_n provide power level signals to controller 230. Controller 230, which may be a dedicated controller or part of a larger system-wide controller, determines weights for gain control of the distribution system. Controller 230 determines weights 214_i to 214_n for individually controlling the gain of signals received over transmission links 204_i to 204_n at multipliers 216_i to 216_n so that the signals do not exceed a predetermined threshold level. The gain controlled signals are then provided to node 226 via transmission links 218_i to 218_n. Node 226 digitally sums the signals from transmission links 218_i to 218_n. Combined power level calculator 228 determines the power level of the signals combined at node 226 and provides the combined power level signal to controller 230. The combined signals are then transmitted upstream via transmission link 231 to DHU or DEU 232. Output signal 227 on transmission link 231 is the combined signals of 218_i to 218_n. If the combined power level calculator 228 sees a combined signal power that exceeds a predetermined level, then the coefficients 214_i to 214_n are set by controller 230 and assigned based on the Power Calculators 208_i to 208_n. The weights of coefficients 214_i to 214_n are set by the controller based on the individual input power calculators 208_i to 208_n. Thus, the smaller the input signal the larger the corresponding weighted coefficient.

[0015] When the signal, 231, at Power Calculator 228 is lower than a predetermined “decay threshold”, the weighted coefficients 214_i to 214_n are increased over time based on a decay time constant and a fair weighting based on input Power calculators 208_i to 208_n is applied to the input signals 204_i to 204_n until a satisfactory power level is achieved at combined power calculator 228. Conversely, when the signal, 231, at combined power calculator 228 is higher than a predetermined “attack threshold,” the weighted coefficients 214_i to 214_n are decreased over time based on an attack time constant and a fair weighting based on input Power calculators 208_i to 208_n is applied to the input signals 204_i to 204_n until a satisfactory power level is achieved at combined power calculator 228.

[0016] Operation of one example of a distributed AGC system according to the present invention will now be described. While the system of FIG. 2 shows only one DEU, many such DEUs may be included in a large distributed communication system.

[0017] The weighted coefficients 214_i to 214_n are initially set to provide a minimum attenuation level of the combined signals on transmission link 231. When the level of the combined signals on transmission link 231 exceeds a predetermined threshold as determined by combined power level calculator 228, controller 230 which signal or signals exceed a threshold level and will attenuate those signals so that the combined power at 228 avoids an overflow condition at the node. Signal levels at each input port 205_i to 205_n are monitored and evaluated by input power calculators 208_i to 208_n to determine whether any transmission link 204i to 204n is providing a signal that exceeds a predetermined level. If one or more of the input power calculators 208_i to 208_n reports a signal level that exceeds the threshold, signals from those input ports will be gain controlled by adjusting weights 214_i to 214_n until all input signal levels are at or below a level such that an overflow condition is avoided. The thresholding and gain control process may be duplicated at other DEUs upstream or downstream from node 226 so Monitoring of the signal levels at the input ports may take place continuously or may be triggered only after combined power calculator 228 senses a combined signal level that warrants a change in gain control of the combined signals at the node. Alternatively, signal levels at the input ports may be polled periodically to determine whether signal levels may have changed.

[0018] In one additional example of the present invention, a frequency selective attenuation device such as an adaptive filter may be used to attenuate only those frequencies at which the offending signals are found. This may be preferable in applications where the interfering signals are confined to specific frequencies and other desirable signals would be lost if the overall gain of signals from one or more DRUs were controlled. In another example, combined power calculator 228 may be eliminated entirely and distributed AGC performed by adjusting the gain of the signals received upstream from the DRUs. Of course, an AGC unit would be required for each DRU and for each signal path in which signal gain might increase by interference, amplification or otherwise.

[0019] A distributed wireless AGC system has been described. The distributed wireless AGC system includes a number of remote units distributed in a coverage area to receive wireless signals in the coverage area, a number of input ports to receive signals from the remote units, input power monitors operatively connected to each of the input ports to determine power levels of signals received at each input port, variable gain controllers to control the gain of signals received at each of the input ports in response to a control signal, a node to combine a plurality of signals from the plurality of input ports, a combined power monitor to determine power levels of the signals combined at the node, a controller to provide control signals to control the gain of each of the variable gain controllers based on a weighting function that is proportional to power received at each input port, as determined by the input power monitors such that the combined power as determined by the combined power monitor does not exceed a predetermined level.

[0020] Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement which is calculated to achieve the same purpose may be substituted for the specific embodiments shown. For example, DHUs and DEUs are not limited to the receipt, gain control of and summing, splitting and transmitting of digitized wireless signals. In some examples of the present invention, DHUs and DEUs are capable of receiving, processing, gain controlling and summing analog wireless signals in addition to or instead of digitized wireless signals. As well, DHUs and DEUs may be capable of splitting, processing and transmitting analog wireless signals in addition to or instead of digitized wireless signals. This application is intended to cover any adaptations or variations of the present invention. Therefore, it is intended that this invention be limited only by the claims and the equivalents thereof.